Method and apparatus for detection of fluid level in a container

ABSTRACT

A fluid level sensor is disclosed having first and second vertically and horizontally nonoverlapping electrode plates for placing on a wall of a fluid container. Most preferably, the plates are also vertically spaced from each other. The capacitor plates are driven by a high frequency square wave. By forming nonoverlapping plates and driving them using a high frequency, the level of a fluid within the container, particularly viscous fluid, is more accurately detected. Control and detection circuitry is also disclosed to trigger an alarm if the fluid level drops below a critical level within the container.

PRIORITY CLAIM

This application claims the benefit of the filing date of U.S.Provisional Patent Application Ser. No. 60/116,302, filed Jan. 19, 1999,for “METHOD AND APPARATUS FOR DETECTION OF A FLUID LEVEL IN ACONTAINER.”

BACKGROUND OF THE INVENTION

The present invention relates generally to determining a level of afluid in a container. More specifically, the invention relates to amethod and apparatus for more accurately sensing when a relatively rapidegress of a viscous fluid, such as blood, reaches a level within thecontainer. The invention is particularly useful in applications whereina reduction in fluid level leaves a film of the fluid on an inner wallof the container.

STATE OF ART

Both rigid and flexible containers are used in many industries to holdand dispense fluids of various natures. Accordingly, fluid level sensorsand corresponding circuitry to indicate the quantity of fluid within acontainer or when the fluid reaches a particular level within thecontainer, such as with a gas tank of a car, are well known.

In the medical industry, both rigid and flexible containers are usedwith a variety of fluid level sensors. Sterile intravenous (“i.v.”) bagsand bottles are commonly used in hospitals to dispense plasma, wholeblood, replacement electrolytes, etc. These containers are usuallylabeled to indicate their contents and volumes. When using suchcontainers, a frequently used procedure is to dispense a metered amountof fluid over a given period of time by unmonitored, gravity-fed dripfeeding. The containers themselves come in different sizes and shapes,and the fluids are administered to patients in widely varying flow rateswhich are often difficult to estimate exactly. Consequently, withoutdirectly monitoring the container throughout its use, it is oftendifficult to determine when all of the fluid within a container has beendispensed.

It is detrimental to a patient to have the flow of an i.v. fluid come toa complete stop, unattended, because of complications which can occurfrom the stop of fluid flow. Complications may include the clogging ofthe needle due to blood clotting, usually requiring reinsertion of a newneedle, or blood passing out of the patient into the tubing. The morefrequently a needle is inserted and reinserted into a vein, the greaterthe risk for complications and infection.

Solutions for medical fluid container monitoring range from complicatedelectronic, motor-driven, peristaltic pump-type systems, which exactlyregulate the fluid flowing from the container to predict when it willrun out, to relatively low-cost sensors which produce an audible alarmwhen the fluid has reached a particular level within its container. U.S.Pat. No. 3,641,543 to Rigby (Feb. 8, 1972) describes a probe-type fluidlevel sensor wherein two probes are placed within the fluid container tomonitor the fluid level based upon the capacitance of the bottle/probessystem. However, a common concern associated with probe-type fluid levelsensors and other sensors which must be placed on the inside of thecontainer involves the risk of introducing contamination into the fluid.For a hospital environment, particularly where fluids generally come inpresterilized containers, introducing a probe into the fluid todetermine its level is a great risk, and providing presterilized sensorsalready within the containers increases healthcare costs and requireshospitals to use common equipment for monitoring the sensors.

Sensors have also been developed for sensing fluid levels from theoutside of a nonconductive container. Three-conductor sensors are shownin both U.S. Pat. No. 3,939,360 to Jackson (Feb. 17, 1976) and U.S. Pat.No. 4,083,038 to Klebanoff (Apr. 4, 1978). For each of these sensorsystems, three conductive strips are placed in a parallel, verticallyand horizontally overlapping arrangement on the side of a rigidcontainer. An associated audible alarm signals when a fluid level withinthe container has dropped below a level determined by the position ofthe sensor on the container. For the three-conductor sensor systems, thealarm is triggered by differences in capacitance between a first and asecond of the conductive strips and the capacitance between the secondand a third of the conductive strips.

U.S. Pat. No. 5,135,485 to Cohen et al. (Aug. 4, 1992), the disclosureof which is hereby incorporated herein by reference, describes anothercapacitance-type fluid level sensing system having a fluid sensorcomprising two conductive strips affixed to a flexible container in aparallel, vertically or horizontally overlapping relationshipsubstantially coextensive with each other. Associated with the fluidsensor is a system of circuitry to produce an alarm signal when fluid inthe container approaches a predetermined level determined by theposition of the fluid sensor on the container and the settings of thecontrol circuitry.

The control circuitry disclosed in Cohen et al. applies a referencevoltage to a first resistor/capacitor combination and to a first inputof a first monostable multivibrator. The control circuitry also appliesthe output of the resistor/capacitor combination to a secondresistor/capacitor combination and to a first input of a secondmonostable multivibrator, and an oscillating wave to a second input ofeach of the monostable multivibrators. At the start of each oscillatorcycle, the monostable multivibrators are triggered and the outputs ofthe multivibrators monitored to determine whether the first input ofeach monostable multivibrator rises to a predetermined level before themultivibrators are triggered again. When the first inputs of bothmultivibrators rise fast enough that their signals exceed apredetermined level before a subsequent triggering, a signal is producedto indicate the fluid level is below a desired level. The flexiblecontainer, fluid and sensor act as the capacitor in the firstresistor/capacitor combination, the response of which adjusts the risetime of the input signal to the first multivibrator as the fluid levelin the flexible container changes. As the volume of liquid within theflexible container decreases, the rise time of the output of the secondmonostable multivibrator increases such that the amplitude of the inputsignal increases with a decrease of the fluid level until the alarmlevel is reached.

U.S. Reissued Pat. 34,073 to Suzuki (Sep. 22, 1992), the disclosure ofwhich is hereby incorporated herein by reference, describes acapacitance-type fluid level sensing system having two conductive stripsaffixed to a flexible container. Suzuki discloses both a horizontallyoverlapping, parallel configuration and a configuration wherein a secondconductor is placed immediately opposite a first conductor on acontainer such that they are in a parallel, vertically overlappingconfiguration, but not immediately adjacent to each other on a commonsurface.

Venous blood containers, which are made of a rigid or flexible resin,are employed in heart-lung bypass circuits used during open heartsurgery. It is critical to monitor the fluid (blood) level in suchcontainers in a manner which provides an accurate and timely signal asto when blood in the container has been reduced below a certain level.While capacitance-type level sensors have been employed in an attempt tomeasure such blood levels, the viscous nature of blood leaves a film onthe interior walls of the container, giving a false level indication.This phenomenon may be exacerbated during the latter stages of emptyinga flexible bag when the inner walls of the bag tend to sag together,trapping the blood film therebetween. It is, therefore, desirable tohave an external fluid level sensor which overcomes the problemsassociated with accurately sensing the levels of viscous fluids in bothrigid and flexible containers.

SUMMARY

The present invention addresses the problems of conventionalcapacitance-type fluid level sensing devices by providing a reliable,relatively simple, capacitance-type level sensor system which issubstantially less susceptible to false level readings attributable tothe presence of a residual film of viscous fluid, such as blood, on aninner wall of a container to which the level sensor of the invention isaffixed.

The system of the invention includes a disposable sensor permanently orremovably placed on or inside a wall of a flexible or rigid,electrically nonconductive container. The sensor comprises twoelectrodes formed of essentially two-dimensional plates of electricallyconductive material deposited on a thin, insulative film backing. Theconductive material and the entire sensor assembly may be made opaque,translucent or transparent, as desired or required for the intendedapplication. Alternatively, the sensor plates may be formed directly onthe material comprising the container wall.

Each of the two plates of the sensor acts as a plate for a capacitor,the fluid inside the container acting as the second plate for eachcapacitor and conducting the electric field between the capacitors. Thecontainer wall acts as a dielectric for the capacitor. As the fluidlevel within the container changes, the capacitance changes slightly.This change in capacitance is detected by control circuitry whichactivates visual and auditory alarms if the capacitance drops below apredetermined level.

The size of the sensor in terms of plate length and vertical as well asany horizontal separation of the plates may be optimized for the systemfrequency and container wall material and thickness, as well as thenature of the fluid, the level of which is to be monitored. The platesof the sensor are arranged with a vertical separation to allow detectionof a rapid decrease of fluid level where a residual layer or film offluid is left on the container walls. Horizontal separation of theplates may be adjusted depending upon the resistance attributable to thefluid film on the interior of the container wall. The sensor is mostpreferably configured so that the capacitor plates are arranged with avertical separation so that an upper plate is completely exposed and thefilm is allowed to dissipate while the fluid is still lowering over asecond, lower plate. The plates may also be horizontally separated by atleast a small distance, or at least not overlap horizontally, tomaximize the film resistance between the sensors. The above-describedsensor and control circuit configuration allows the level detection ofblood and other conductive fluids that leave a conductive film on thecontainer wall.

The system further includes a control box housing detection and controlcircuitry which is attached to the sensor with a connector and flexiblecable. The connector may be a Zero Insertion Force (ZIF) connector asknown in the art. If desired, the cable and connector may be made partof a sensor assembly and disposable therewith. The control boxincorporates both audio and visual alarms to indicate that the sensor isnot connected to the control box as well as if the sensor does notdetect the fluid (i.e., the fluid level is below the sensor).

As noted previously, many fluids, particularly those of substantialviscosity, leave thin conductive films on the container walls that maytake seconds, or even minutes, to thin to a level where a conventionalcapacitor-type level sensor may detect the change. To be able toaccurately detect rapidly changing fluid levels in spite of the thin,conductive fluid film left on the container walls, several things shouldhappen. First, as noted above, it is desirable to drive the sensorcapacitance with a high frequency of about 4 MHz or greater. At lowerfrequencies, for example, 1 MHz, a residual film of blood on a containerwall may give a false level reading, indicating erroneously that theblood level within the container is at or higher than the level of thesensor. The thin, conductive fluid film may be characterized as smallcapacitive electrodes extending through the resistive portion of thefilm. The film resistance and capacitances together act as a low passfilter. Therefore, increasing the frequency reduces the effect of thecapacitance in the residual film adjacent the sensor.

The control box circuitry is configured to detect a change incapacitance of the sensor by applying a high-frequency, preferably atleast about 4 MHz, and most preferably at least about 8 MHz, square waveto a series resistor and capacitor network. The resistor is located inthe control box and the capacitor network is provided by a combinationof the cable capacitance and the sensor capacitance. This arrangementand the driving signal result in a substantially triangular wave outputat the capacitor which is amplified and monitored for a small change inamplitude resulting from a change in the network capacitance due to afluid level change within the container.

The frequency of the square wave input signal has been selected byexperimentation to minimize capacitance detected attributable to a thinlayer of fluid remaining on the side walls of the container as the fluidlevel lowers past the sensor. When the capacitance lowers due to thefluid level lowering past the sensor, a comparator detects the amplitudechange and filters the detection noise and produces visual and audioalarms.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram of a sensor system according to the presentinvention;

FIG. 2 is a diagram of a fluid level sensor according to an embodimentof the present invention;

FIG. 3 is a block diagram of an embodiment of the control circuitry ofthe present invention;

FIG. 4 is a timing diagram relating to signals within the controlcircuitry of the present invention; and

FIGS. 5 a–5 c are diagrams of a sensor and fluid container wallaccording to embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a fluid level sensor system 2 according to an embodimentof the present invention including a fluid container 4, a fluid levelsensor 6, and detection and control circuitry housed in a control box 8.The fluid container 4 may be any fluid container having an electricallynonconductive wall such as both the rigid- and flexible-walled fluidcontainers commonly used in medical applications for storing blood,saline solution, human waste, or other fluids or mixtures. The fluidlevel sensor 6 may be affixed to a side of the fluid container 4 by anadhesive such as a pressure sensitive adhesive or other medical gradeadhesive well known to one of ordinary skill in the art. The fluid levelsensor 6 may be affixed to an outside wall of the fluid container 4 bymedical personnel or other operator just before use, or formed on orwithin the fluid container wall as part of a manufacturing process andsold as an integral unit with the fluid container 4.

A conductive cable 10, such as a coaxial cable or other two-signalcable, couples the fluid level sensor 6 to the detection and controlcircuitry within the control box 8 and may be formed as part of thefluid level sensor 6, as part of the detection and control circuitry, oras a separate part to couple to both the fluid level sensor 6 and thedetection and control circuitry prior to use. Most preferably, the cable10 is coupled to the fluid level sensor 6 through a Zero Insertion Force(“ZIF”) connector as is known in the art.

As illustrated in FIG. 1, the detection and control circuitry mostpreferably includes an external switch for controlling the fluid levelsensor system 2 between On 12 and Off 14 states. Although the switchingdevices 12 and 14 of FIG. 1 are shown as separate switching devices,other switching devices known in the art, such as asingle-pole-double-throw switch, may serve the same function ofactivating and deactivating the circuitry. The detection and controlcircuitry also includes external alarm indicators 16 and 18 to enableboth a visual alarm 16 and an audible alarm 18 from within the controlbox 8. Additionally, the detection and control circuitry may includeother control devices such as buttons to deactivate an audible alarm orreset a system, and other indicators such as a display to indicate amore precise fluid level.

FIG. 2 is a diagram of an embodiment of the fluid level sensor 6 of thepresent invention comprising a thin, electrically insulative film 20, apair of conductive plates 22 and 24, and conductive traces 26 and 28extending from each of the conductive plates 22 and 24 to a terminalarea 35 to facilitate easier coupling between the conductive plates 22and 24 and the detection and control circuitry 36 (see FIG. 3). The twoconductive plates 22 and 24 of the fluid level sensor 6 are preferablyvertically spaced and horizontally nonoverlapping, or are offset platesof conductive material such as gold, silver, copper, aluminum, or othernonmetallic conductor. Most preferably, an opaque or transparentconductive material such as screened silver or gold, or indium tin oxide(“ITO”) such as is commonly used in electronic displays, is used to formthe conductive plates 22 and 24 so that the conductive plates do notblock an operator's view of the fluid within the container. The thin,electrically insulative film 20 may be formed of any insulating materialsuitable as a dielectric for a capacitor. An example of a suitable filmincludes Mylar™. As required by a particular application, the conductiveand nonconductive materials comprising the fluid level sensor 6 may bemade opaque, translucent or transparent. A connector insertion pointindicator 34 is also preferably applied to the insulative film 20 toindicate to the user when the terminal area 35, including the enlargedtrace ends, has been fully inserted into a connector socket.

Although vertically or horizontally overlapping or nonoffset plates willfunction as a sensor, to the extent the sensor plates overlap or are notoffset, the rate at which the circuit detects rapidly lowering fluidlevels is significantly decreased. This decrease in performance isexperienced because the fluid film remaining on the container wallcontinues to conduct more through the overlapping area. Experimentationhas shown the best results occur where there is little or substantiallyno overlapping because the signal must travel a longer path through thefluid film.

The two conductive plates 22 and 24 operate, in conjunction with thefluid within the fluid container 4 (see FIG. 1), as two capacitors inseries, each conductive plate 22 and 24 forming a capacitor with thefluid in the fluid container 4 using the fluid container wall as adielectric. As the fluid level within the fluid container 4 changes, theconduction between the plates change, causing a difference in thedetected capacitance of the plate/fluid system changes.

A difficulty experienced with systems using vertically or horizontallyoverlapping conductive plates is that as the fluid level within thecontainer decreases, there may be residual fluid film left on the wallsof the container. For highly viscous fluids such as blood, the residualfluid left on the walls of the container may take a few seconds or evena few minutes to flow off. For fluid level sensors employinghorizontally or vertically overlapping or partially overlapping plates,detection of the fluid level within the fluid container is more delayedthan with nonoverlapping plates. The nonoverlapping arrangement of theconductive plates 22 and 24 assists in a more rapid and accuratedetermination of when the fluid level within the fluid container 4 hasreached a critical range near the sensor level. Arranging the conductiveplates 22 and 24 with a larger vertical separation or spacing 32 enablesan easier detection of a rapid decrease of fluid level where a residuallayer or film of fluid is left on the container walls. Arranging theconductive plates 22 and 24 with a larger horizontal separation orspacing 30 adjusts for the system resistance attributable to the fluidfilm on the interior of the container wall. The amount of separationplaced between the conductive plates 22 and 24 is limited, however, bythe fluid level range within which an indication is acceptable. Forexample, by increasing the vertical separation 32 between the conductiveplates 22 and 24, the accuracy of the sensor indication becomes moreresistant to the effects of a viscous film, but the range within whichthe indication may initiate also increases. For the conductive platearrangement illustrated in FIG. 2, vertical spacing 32 changes had agreater effect on sensor accuracy than did horizontal spacing 30 changesbecause the fluid flow level decreased along a vertical axis.

The dimensions of the conductive plates 22 and 24 in terms of platethickness, width and length, as well as their horizontal 30 and vertical32 spacing from each other, may be optimized for the system frequency,container wall material and thickness, as well as the nature of thefluid, positioning of the sensor and quantity of fluid to be monitored.The thickness and, to a lesser extent, the material of the container arelargely determinative of the conductive area required. The verticalseparation 32 of the conductive plates 22 and 24, one plate above theother, encourages the fluid film over the upper plate to thin out as thefluid is lowering over the lower plate. The horizontal separation 30, ifany, of the conductive plates 22 and 24 increases the resistance betweenthe plates attributable to the residual film on the inside of thecontainer wall. Relatively larger or longer electrode plates are moredesirable for use with relatively rigid containers with their thickerwalls to increase the relatively lower capacitance associated therewith.Relatively smaller or shorter electrode plates appear to producesuperior results on flexible-walled containers. It is believed that oneof ordinary skill in the art is sufficiently familiar withcapacitor-type fluid sensor design techniques to design a suitable fluidlevel sensor 6, given the characteristics of the particular system forwhich the fluid level sensor 6 will be used. The conductive plates 22and 24 are each coupled to detection and control circuitry 36 (FIG. 3)through conductive traces 26 and 28 secured to the thin, electricallyinsulative film 20 and extending to a terminal area 35.

FIG. 3 illustrates a block diagram of detection and control circuitry 36according to an embodiment of the present invention. The detection andcontrol circuitry 36 is configured to detect a change in the capacitanceof the sensor by applying a high-frequency signal from oscillator 38 atpoint A, preferably at least about a 4 MHz, and most preferably at leastabout an 8 MHz, square wave 58 (see FIG. 4) to a series resistor 40,coupled to a first of two terminals 42 for connection to a fluid levelsensor 6 such as that illustrated in FIG. 2. The second of the twoterminals 42 may be connected to a reference voltage such as ground.From the combination of the R-C constant effects of the series resistor40, and the capacitance caused by the fluid level sensor and sensorcable attached to the two terminals 42, the signal B at the junction ofthe series resistor 40 and the first of the two terminals 42approximates a triangular wave signal 60 (see FIG. 4). The triangularwave signal 60 (FIG. 4) is input into an amplifier 44, such as anoperational amplifier, to boost the signal. The boosted signal is thenfiltered by a reference filter 46 to give a relative amplitude of thesignal. The boosted signal is also sent through a detector 48 toestablish a DC reference voltage to act as a threshold for the sensoralarm. The outputs of both the reference filter 46 and the detector 48are compared using a threshold comparator 50 to determine whether thereference filter 46 output signal exceeds the DC reference voltagethreshold. When the output of the detector 48 exceeds the DC referencevoltage threshold, the threshold comparator 50 output signal at point Cdrops low as shown at 62 in FIG. 4, indicating to the alarm andindicator driver circuit 56 that an alarm should be initiated. Alarm andindicator driver circuit 56 initiates the visual indicator alarm 54 andthe auditory alarm 52 in response to the threshold comparator 50 outputgoing low.

FIG. 5 a depicts a fluid container wall 64 having a fluid sensor 66affixed to an external surface of the wall 64. The fluid sensor 66includes a thin, electrically insulative mounting structure 68, firstand second electrodes 70 and a terminal area 72 to which controlcircuitry may be coupled. FIG. 5 b depicts a fluid sensor 74 wherein thefluid sensor electrodes 76 are placed within the fluid container wall78. The fluid sensor electrodes 76 may be placed within the fluidcontainer wall 78 by forming the electrodes 76 on a surface of, oraffixing the electrodes 76 to a surface of, one of two flexible or rigidwall sheets 80, and then affixing the wall sheets 80 to each other.Conductors 82 from each of the electrodes 76 may extend between the wallsheets 80 to a container wall exit point and terminal area or, as shownin FIG. 5 c, may extend through the container wall 86 to an externalsurface of the container for coupling to control circuitry. FIG. 5 cillustrates a sensor 84 affixed to an internal surface of a fluidcontainer wall 86. The sensor in the embodiment of FIG. 5 c includes athin, electrically insulative film 88 isolating sensor electrodes 90from an internal volume of the fluid container. In this way, when thefluid container is filled with fluid, the electrodes are not shorted bythe fluid. The thin, electrically insulative film 88 acts as thedielectric for the sensor capacitor in this embodiment. Conductors 92may extend through the fluid container wall 86 immediately behind theelectrically insulative film 88 to allow for coupling with controlcircuitry while minimizing the possibility of contamination with theinternal volume of the fluid container. Alternatively, conductors mayextend along an inside surface of the fluid container wall 86 and exitthe container at another location.

While the system is preferably powered with a conventional nine voltbattery, other power sources could be easily adapted.

Obviously, the fluid-level sensor shown and described with reference toFIG. 2, though particularly useful for detecting a lowering level ofviscous fluid within a container, will also function for a rising levelof fluid. One of ordinary skill in the art will understand the minorcircuit modifications required to enable the sensor to detect and alertto fluid rising to a predetermined level.

It should be noted that for many control circuits known in the art,temperature compensation relative to ambient temperature (≈22° C.) maybe necessary to ensure accurate level detectin. Since the temperature ofthe environment in which the system of the invention is employed mayvary significantly, particularly in less-developed regions of the worldwhere operating rooms are not climate controlled, such temperaturecompensation is believed to be a significant feature of the system. Itshould also be noted that bloods with higher hematocrits (red blood cellpercentages) are more viscous and thus more likely to fail to initiate alow fluid level signal if the electrode plates are not appropriatelysized and spaced.

While the invention has been described in terms of a preferredembodiment, it will be understood and appreciated by those of ordinaryskill in the art that it is not so limited. Many additions, deletionsand modifications to the embodiment disclosed herein may be made withoutdeparting from the scope of the invention.

1. A capacitive sensor for detecting a level of a viscous fluid in acontainer having an interior volume, the sensor comprising mutuallycooperative, mutually electrically isolated first, upper and second,lower electrodes arranged for placement on a wall of the container inisolation from the interior volume of the container, wherein each of thefirst and second electrodes exhibits a two-dimensional area having avertical dimension and a horizontal dimension, and wherein the first andsecond electrodes are arranged in mutual proximity such that at least amajority of each of their respective areas are both vertically andhorizontally offset from each other and to an extent at least sufficientto enable rapid detection of a decreasing level of the viscous fluid inthe container when the viscous fluid has reached a level proximate alower edge of the first, upper electrode and a residual film of theviscous fluid remains on an inner surface of the wall of the containerabove the level of the viscous fluid and adjacent at least a portion ofthe first, upper electrode.
 2. The sensor of claim 1, wherein the firstand second electrodes are arranged such that their respective areas aresubstantially both vertically and horizontally offset from each other.3. The sensor of claim 1, wherein the first and second electrodes arearranged such that their respective areas are completely both verticallyand horizontally offset from each other.
 4. The sensor of claim 1,wherein the first and second electrodes are both vertically andhorizontally spaced from each other.
 5. The sensor of claim 1, whereinthe electrodes comprise substantially two-dimensional plates.
 6. Thesensor of claim 1, further comprising a conductor coupled to each of thefirst and second electrodes.
 7. The sensor of claim 6, wherein theconductors coupled to each of the first and second electrodes are alsocoupled to control circuitry.
 8. The sensor of claim 7, wherein theconductors coupled to each of the first and second electrodes arecoupled to the control circuitry through a Zero Insertion Forceconnector.
 9. The sensor of claim 1, further comprising controlcircuitry, wherein the control circuitry is coupled to one of the firstand second electrodes and configured to supply an oscillating signalhaving a frequency greater than 1 MHz thereto, another of the first andsecond electrodes being coupled to a reference voltage.
 10. The sensorof claim 9, wherein the control circuitry is configured to supply asignal at a frequency of at least about 4 MHz.
 11. The sensor of claim10, wherein the control circuitry is configured to supply a signal at afrequency of at least about 8 MHz.
 12. The sensor of claim 1, furthercomprising control circuitry coupled to one of the first and secondelectrodes and configured to detect a change in a capacitance of thesensor.
 13. The sensor of claim 1, further comprising at least one alarmresponsive to an output signal of the sensor.
 14. The sensor of claim 1,wherein the first and second electrodes are horizontally spaced.
 15. Thesensor of claim 1, wherein the first and second electrodes are arrangedfor placement on a wall of the container.
 16. The sensor of claim 15,further comprising a mounting structure to which the first and secondelectrodes are affixed.
 17. The sensor of claim 16, wherein the mountingstructure is a thin, electrically insulative film.
 18. The sensor ofclaim 17, wherein the thin, electrically insulative film is Mylar. 19.The sensor of claim 1, wherein the first and second electrodes areplaced within the wall of the container.
 20. A method for detecting alevel of a viscous fluid within a container having an interior volume,comprising: placing a capacitive structure including mutuallycooperative, mutually electrically isolated, first, upper, and second,lower electrodes on a wall of the container in isolation from theinterior volume of the container, wherein each electrode exhibits atwo-dimensional area having a vertical dimension and a horizontaldimension and wherein the first and second electrodes are arranged inmutually proximity such that at least a majority of each of theirrespective areas are both vertically and horizontally offset from eachother; driving the capacitive structure with an oscillating signal at afrequency of more than about 1 MHz and generating an output signal fromthe capacitive structure responsive thereto; decreasing a fluid levelwithin the container at a rate sufficient to leave a residual film ofthe viscous fluid on an interior surface of the wall above the level ofthe viscous fluid and at least proximate a lower edge of the first,upper electrode; and rapidly detecting a change in the output signalresponsive to the decreasing of the fluid level.
 21. The method of claim20, wherein placing the capacitive structure on a wall of the containercomprises placing the capacitive structure within the wall of thecontainer.
 22. The method of claim 20, wherein driving the capacitivestructure with an oscillating signal at a frequency of more than about 1MHz further comprises driving the capacitive structure at a frequency ofat least about 4 MHz.
 23. The method of claim 20, wherein driving thecapacitive structure with an oscillating signal at a frequency of morethan about 1 MHz further comprises driving the capacitive structure at afrequency of at least about 8 MHz.
 24. The method of claim 20, whereinplacing the capacitive structure on a wall of the container comprisesforming the capacitive structure on a mounting structure and affixingthe mounting structure to an exterior wall of the container withadhesive.
 25. The method of claim 20, wherein placing the capacitivestructure on a wall of the container comprises forming the capacitivestructure on the wall.
 26. The sensor of claim 20, further comprisingdetermining whether the output signal exceeds a reference signal. 27.The method of claim 26, further comprising initiating at least one alarmif the output signal exceeds the reference signal.
 28. The method ofclaim 27, wherein the at least one alarm is at least one of an audiblealarm and a visual alarm.